Digital repeater system

ABSTRACT

A digital repeater system for repeating RF signals comprises: a receiving section for receiving an RF input signal, the RF input signal comprising at least one frequency band including a multiplicity of subbands associated with a multiplicity of communication channels; and at least one transmitting section for transmitting the RF output signal. The receiving section is constituted to digitize the RF input signal to obtain a digital input signal and to isolate, within the digital input signal, the multiplicity of subbands from each other to obtain a multiplicity of digital subband signals. The at least one transmitting section is constituted to combine the digital subband signals to obtain a digital output signal and to convert the digital output signal to an RF output signal. In addition, the receiving section comprises a power profile estimation unit for determining a power estimate for each digital subband signal associated with the multiplicity of subbands and a muting device for muting a digital subband signal of a particular subband based on the power estimate. In this way a digital repeater system for repeating RF signals is provided which allows for a detection of unused portions of a frequency band in order to improve the performance of the overall system.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Stage Application of PCT ApplicationSerial No. PCT/US2015/024685, filed Apr. 7, 2015, and titled “DIGITALREPEATER SYSTEM,” which claims the benefit of EP Patent ApplicationSerial No. 14163855.1, filed Apr. 8, 2014, and titled “DIGITAL REPEATERSYSTEM,” the contents of all of which are hereby incorporated byreference.

BACKGROUND

This application relates to a digital repeater system for transmittingsignals and to a method for operating a digital repeater system.

A digital repeater system of this kind typically comprises a receivingsection for processing a digital input signal, the digital input signalcomprising at least one frequency band including a multiplicity ofsubbands associated with a multiplicity of communication channels. Thedigital repeater system furthermore comprises at least one transmittingsection for transmitting an output signal. The receiving section isconstituted to isolate, within the digital input signal, themultiplicity of subbands from each other to obtain a multiplicity ofdigital subband signals. The at least one transmitting section isconstituted to combine the digital subband signals to obtain an outputsignal for transmission.

A digital repeater system of this kind may be a classical “off-air”repeater which receives signals via an air interface, operates on thereceived signals and transmits the processed signals again via the air.A digital repeater system of this kind however may in general be asystem using any kind of donor input and transport medium connecting areceiving section and a transmitting section.

A digital repeater system in this regard may also be a distributedsystem, such as a distributed antenna system, in which signals aredistributed to multiple distributed antenna units for transmission (in adownlink direction) or are received by the distributed antenna units andcombined (in an uplink direction).

EP 1 324 514 A2 discloses a repeater system for transmitting receivingRF signals to and from an area obstructed by a mountain, a building orthe like. One antenna is linked to for example a base station antenna ofa cell in a cellular network, and another antenna is directed towardsthe obstructed area. Signals are received and digitally separated into anumber of different frequency channels. The separated channels are thenprocessed and recombined for transmission into the obstructed area tomobile users located in that obstructed area. For separating the RFsignals into the different frequency channels, a channelizer is providedwhich comprises a multiplicity of digital downconverters and amultiplicity of digital upconverters.

EP 1 109 332 A2 discloses a digital repeater for receiving andretransmitting radiofrequency (RF) signals in which a first RF signal isdownconverted to an intermediate-frequency (IF) signal, converted into adigital signal and processed with a digital signal processor. Uponamplification the digital signal is amplified and converted into ananalog signal for retransmission.

EP 1 303 929 B1 discloses a digital repeater system in which a host unitis connected via a transport medium (such as optical fibers) to amultiplicity of remote units. The remote units for example may bedistributed throughout a building. In a downlink direction, the hostunit digitizes an incoming RF signal and transmits the digitized RFsignal via the transport medium to the remote units for retransmission.In an uplink direction, the remote units receive an incoming RF signal,digitize it and transmit it to the host unit for transmission towards anoutside communication network.

Nowadays, repeater systems are constituted to serve a wide frequencyband, for example a complete 3GPP band. Herein, a frequency bandtypically contains multiple carriers carrying information according todifferent communication technologies and standards such as GSM, UMTS,LTE or the like.

Within such repeater systems, the full spectrum of the supported bandmay not be used at all times. If, however, signals within a subband areprocessed and transported in a repeater system which currently do notcontain useful information, but represent only noise, this may degradeperformance of the system and may add to the overall noise of theretransmitted RF signals.

There hence is a desire to detect unused spectrum within a supportedfrequency band in order to be able to adapt the processing of thefrequency band for an optimization of the system behaviour.

EP 0 681 374 A1 discloses an analog repeater which processes analogsignals in an uplink and a downlink direction in a channelized fashion.Herein, the different channels may be muted by evaluating an uplinkspectrum.

SUMMARY

One embodiment is directed to a digital repeater system for repeating RFsignals and a method for operating such a digital repeater system whichallow for a detection of unused portions of a frequency band in order toimprove the performance of the overall system.

In one embodiment, the repeater system comprises

-   -   a power profile estimation unit for determining a power estimate        for each digital subband signal associated with the multiplicity        of subbands and    -   a muting device for muting a digital subband signal of a        particular subband based on the power estimate.

By means of the power profile estimation unit in particular a powerestimate of a digital subband signal associated with a particularsubband may be determined as a function of time such that a momentarypower estimate for a digital subband signal of a particular subband isobtained. Dependent on the power estimate, then, a digital subbandsignal may be muted by means of the muting device if it is detected thatthe power estimate is low and hence the digital subband signal is foundto contain little power. For this, the power estimate is for examplecompared with a threshold and, if it is found that the power estimate issmaller than the threshold, it is assumed that the subband contains nosignals carrying information, but only noise. In that case, the digitalsubband signal of the particular subband is muted and hence is notprocessed by the transmitting section for retransmission.

The muting decision may be based on the comparison of a momentary powerestimate of an overall subband signal to a fixed threshold, wherein thethreshold may also be dynamically adjustable. The muting decisionhowever may also be a complex decision based for example on detecting apower estimate of interfering signals in a subband, intermodulationproducts or the like in order to mute signals relating to suchinterfering signals of intermodulation products.

In this regard it shall be noted that the power profile estimation unitand the muting device may be located anywhere in the repeater system.The power profile estimation unit and the muting device may inparticular be part of the receiving section, but may also be part of thetransmitting section.

In addition, the receiving section and the transmitting section may beimplemented as separate units, but also may be part of a single unit.For example, in a classical “off-air” repeater a receiving section and atransmitting section may be enclosed in a single repeater unit. In adistributed repeater system, in contrast, a host unit and multipleremote units connected to the host unit via a suitable transport medium,such as an electrical or optical transport medium for analog or digitaltransmission, may be present, the host unit for example comprising areceiving section (for reception in the downlink direction) and atransmitting section (for transmission in the uplink direction) andlikewise each remote unit for example comprising a transmitting section(for transmission in the downlink direction) and a receiving section(for reception in the uplink direction).

Also, functions of the receiving section and/or functions of thetransmitting section may be distributed over multiple units. Forexample, in the uplink direction signals may be received at remote unitsand may be aggregated for transport to an intermediate unit. In theintermediate unit the aggregated signal may be processed and may betransported to a (separate) host unit for transmission via the hostunit. Functions of the receiving section in this example are taken overby the remote units in connection with the intermediate unit.

The receiving section may, in one embodiment, be constituted to receivean RF input signal and to digitize the RF input signal to obtain thedigital input signal. Vice versa, the at least one transmitting sectionmay be constituted to combine the digital subband signals to obtain adigital output signal and to convert the digital output signal to an RFoutput signal for transmission. The repeater system hence is constitutedto digitize a received RF signal, to process it in a digitized fashionand to retransmit it as an RF signal.

In another embodiment, a digital input signal may also be directly fedto the receiving section via a digital link and/or a digital outputsignal may be transmitted to a digital network via a digital link. Suchan embodiment may for example embody a distributed antenna system (DAS).

By determining the power estimate and by comparing the power estimatefor a particular subband for example to a (pre-determined) threshold itcan be made sure that only such signals with a sufficientsignal-to-noise ratio (SNR) are processed and transported in the digitalrepeater system. For example, only signals having an SNR larger than 6dB may be transported, whereas all other signals are muted.

The threshold in this regard may be fixed or may dynamically be adjustedbased on for example on a power distribution associated with the(broadband) input signal from which the (narrowband) subband signals arederived.

Muting a digital subband signal in the context of this text is to beunderstood as cancelling the signal such that it does not add to theretransmitted RF signal. For this, the digital subband signal may becancelled out, i.e. all digital sample values are set to zero.

Within the digital repeater system the digital subband signals of thedigital subbands of the overall frequency band are processed in anisolated fashion. For separating the digital subband signals from eachother, the receiving section for example comprises a multiplicity ofdemodulators for demodulating the digital input signals into thebaseband to obtain digital subband signals located in the baseband. Onedemodulator is associated with one subband and demodulates the digitizedRF input signal such that the digital subband signal associated with thesubband is obtained.

The subband signals generally may overlap in the frequency domain. Bymeans of the demodulators the separate subband signals are extractedfrom the overall signal such that separate subband signals forprocessing in the repeater system are obtained.

The demodulator outputs advantageously a complex digital subband signalhaving an in-phase component and a quadrature component, i.e. signalswith the same frequency and amplitude, but being 90° out of phase.

The power profile estimation unit may, in that case, in an easy fashiondetermine a momentary power estimate at a particular digital sample timeby adding the squared digital sample values of the in-phase componentand the quadrature component. From the in-phase component and thequadrature component the power estimate hence may be obtained in acomputationally efficient manner by simply adding the squared samplevalues of the in-phase and the quadrature components of the digitalsubband signal. By determining a power estimate for each digital subbandsignal as a function of time power estimate profiles for the varioussubbands are obtained such that a muting decision may be taken in atime-dependent manner based on the time-dependent power estimateprofiles.

In order to isolate the digital subband signals from each other, thereceiving section in one embodiment comprises a multiplicity of digitalfilters for digitally filtering the digital subband signals. The digitalfilters, in particular, are constituted as lowpass filters havingcut-off frequencies corresponding to the bandwidths of the differentsubbands. Herein, the lowpass filters do not necessarily have the samecut-off frequency, but the cut-off frequencies of the digital filtersmay differ dependent on the particular subbands to be processed by therepeater system. The filter coefficients of the digital filters hereinmay be adjusted to adapt the repeater system to the particular subbandsto be processed by the system.

Both the demodulators and the digital filters may be configurable bysoftware such that the repeater system may be adjustable to processdifferent subbands and bands. Hence, the repeater system can be adaptedin its channel setup such that, depending on which subbands one ormultiple operators wish to repeat, the demodulators and filters can beconfigured to process a particular set of subbands at a particular setof carrier frequencies and having a particular set of bandwidths(wherein the bandwidths of the different subbands may differ).

A subband may correspond to a particular communication channelassociated with a carrier at a particular carrier frequency. Or asubband may be associated with multiple communication channels havingcarrier frequencies within the frequency range of the subband.

In this regard it is to be noted that a subband may correspond to aparticular communication channel or a multiplicity of particularcommunication channels. This however is not necessary. It also isconceivable that the frequency band is divided, by means of the filters,into (overlapping or non-overlapping) subbands which do not correspondto actual communication channels, but merely represent frequencyportions of the overall frequency band which may be recombined afterprocessing to again obtain the overall signal of the entire frequencyband. In this way for example broadband signals such as signals using aspread spectrum code multiplexing technique (e.g., as in CDMA or UMTSsystems) may be (artificially) split into subbands of a smallerbandwidth, which then may be processed separately and afterwardsrecombined.

The subbands each may for example have a narrow bandwidth of for examplein between 150 kHz and 1 MHz, for example between 180 kHz and 400 kHz.The subbands may all have the same (narrow) bandwidth, or the subbandsmay have different bandwidths. The digital repeater system may forexample be constituted to process narrowband digital subband signalswithin a complete (uplink or downlink) 3GPP band.

By means of the digital filters all such portions of the output of thedemodulators are cut off which do not belong to the various subbandssuch that the subband signals are isolated from each other in thebaseband.

After passing the digital filters the digital subband signals may be fedto downsamplers for decimating the sampling rate of the digital subbandsignals. The power profile estimation unit is arranged after thedownsamplers and acts onto the downsampled digital subband signals. Thein-phase components and the quadrature phase components of the complexdigital subband signals herein may be decimated to a minimum acceptablevalue for the signals in the baseband.

The decision whether to mute a digital subband signal or not is takendependent on the power estimate derived from the (complex) digitalsubband signal associated with the particular subband. To mute thedigital subband signal of a particular subband the muting device forexample may comprise a switch for switching-off the digital subbandsignal for example if the power estimate for the particular subband isfound to be smaller than a predetermined threshold or if interferingsignals or intermodulation products are detected.

The switching-off of a digital subband signal may be implemented inhardware or software. For example, the switching-off of a digitalsubband signal may be implemented by setting all sample values of thedigital subband signal to zero.

The muting device may be configured to take a muting decision based onone power estimate derived from the (complex) digital subband signal ata particular sampling time. Hence, for each sampling time a powerestimate for the digital subband signal is determined and based on thispower estimate it is decided whether to mute a channel or not. Themuting decision hence is taken on a sample-by-sample basis and hence maychange with time in a rather fast way.

In one embodiment, the muting device may for example be configured totake a muting decision based on a power estimate derived from the(complex) digital subband signal periodically after a predefined numberof sampling times, for example at every second or third sampling time.The muting decision is hence not taken at every sampling time, but at apredefined temporal distance corresponding to the time period betweenfor example two or three samples.

In one embodiment, the receiving section is connected to thetransmitting section via a transport medium, such as an air interface orone or multiple optical fibers. Via the transport medium the severaldigital subband signals associated with the separate subbands aretransported to the transmitting section, which may be remote from thereceiving section. Herein, prior to transmitting the digital subbandsignals via the transport medium, the digital subband signals which arenot muted and which shall be transported may be fed to an aggregationunit for aggregating the signals to be transported via the transportmedium. Via the aggregation unit the digital subband signals may becombined into a combined signal to be transported via the transportmedium, wherein on the side of the transmitting section the transportedsignal may again be separated into the different digital subband signalsand may be fed to modulators of the transmitting section for modulatingthe digital subband signals into an intermediate frequency band. Themodulated signals are then fed to a combiner to combine the modulateddigital subband signals to obtain the digital output signal fortransmission from the transmitting section.

The muting decision may also be fed to the aggregation unit of thereceiving section such that, based on the muting decision, also theaggregation unit and transport medium may reconfigure themselves toavoid transporting any muted signals. Hence, if a digital subband signalis muted, this signal will not be transported via the transport medium.

The power profile estimation unit and the muting device herein may taketheir muting decision in a rather fast manner, for example, after apower estimate for a sample of the (complex) digital subband signal isdetermined. In contrast, the aggregation unit may take a decision forreconfiguring the transport network in a slower manner, such that areconfiguration takes place not after each power estimate for eachsample, but in a slower manner, for example only after a decision tomute a particular digital subband signal or not does not change for apre-determined number of samples.

A system and method for aggregating signals to be transported via atransport medium is for example described in co-pending international(PCT) application with application number PCT/US2015/018922, which shallbe incorporated by reference herein.

Background of this is that, in particular in the uplink direction, thecombination of broadband signals received from multiple remote units ata host unit may lead to an increase of the noise level in the combinedsignal and hence to a degradation in the signal-to-noise ratio (SNR) ofthe combined signal. This is due to the fact that any broadband signalfrom any remote unit contains noise, which adds up upon combining thedifferent signals from the different remote units. Such degradation ofthe SNR is disadvantageous and hence shall be avoided or at leastreduced.

By splitting up a broadband input signal, in particular in the uplinkdirection, into several (narrowband) subband signals and by processingthe subband signals separately the effect of SNR degradation may atleast be reduced. By selecting only those subband signals which includeinformation (which is judged by the power contained in the signal) fortransport via the transport medium and by combining only the useful,information-containing signals of one or multiple remote units at thehost unit, it can be avoided that unnecessary noise (i.e., noise ofsubband signals containing no information) adds to the noise of thecombined signal transmitted by the host unit for example to one ormultiple base stations connected with the host unit.

The aggregation unit hence is configured to select only a subset of thesubband signals of a remote unit, corresponding to the unmuted signals,for transport via the transport medium. The selection may then also betaken into account at the host unit for combining the subband signals ofone or multiple remote units into a combined output signal for routingto one or multiple base stations such that only those subband signalsare combined which contain useful information.

The host unit in this regard may also be constituted to take, forcombining the subband signals, a mapping to antenna ports of one ormultiple base stations into account. By combining only those subbandsignals (of one or multiple remote units) which shall be transmitted toa particular antenna port of a particular base station into a combinedsignal it can be obtained that only the noise of the subband signals tobe routed to the particular antenna port in the uplink direction addsup, leading to an improvement of the SNR in the combined signal.

In one embodiment, the digital repeater system comprises a host unit andone or multiple remote units connected to the host unit. The host unitmay for example be located outside a building, whereas the remote unitsare distributed over different floors of the building to providecoverage within the building.

Both the host unit and the remote units may comprise a receiving sectionand a transmitting section of the kind described above. Namely, in adownlink direction the receiving section of the host unit receives an RFsignal from one or multiple base stations of one or multiplecommunication networks via an air interface, processes the RF signal andprovides it to the transmitting sections of the different remote unitsfor retransmission via the remote units. In the uplink direction, incontrast, the receiving section of a remote unit receives an RF signalfrom inside the building and provides the RF signal, after processing,to the transmitting section of the host unit for transmission towards anoutside communication network, in particular for routing to antennaports of one or multiple base stations of one or multiple communicationnetworks of equal or different radio access technologies.

In a particular example, the functions of the receiving section and thetransmitting section may also be distributed over the host unit and theone or multiple remote units or one or multiple intermediate unitslocated in between the host unit and the remote units. For example, inthe uplink direction an RF signal may be received at one or multipleremote units and may be forwarded to an intermediate unit located in thevicinity of the host unit. In the intermediate unit the signals receivedfrom the remote units are processed and aggregated and provided to thehost unit for transmission to an outside network. In this example thepower profile estimation unit and the muting device may for example belocated in the intermediate unit such that a majority of the processingon the receiving side is carried out in the intermediate unit.

The digital repeater system hence provides a distributed coveragesolution by means of which an RF signal comprising a frequency bandincluding various subbands associated with various radio technologiessuch as a GSM, UMTS, LTE or the like, for example all signals in a 3GPPfrequency band, may be distributed via a host unit to multiple remoteunits providing coverage in an obstructed area. The remote units may beconnected to the host unit for example via a network of optical fibersor via an air interface or another RF connection means.

Another embodiment is directed to a method for operating a digitalrepeater system for transmitting signals. In the method,

-   -   a receiving section processes a digital input signal, the        digital input signal comprising at least one frequency band        including a multiplicity of subbands associated with a        multiplicity of communication channels, and    -   at least one transmitting section transmits an output signal,        wherein the receiving section isolates, within the digital input        signal, the multiplicity of subbands from each other to obtain a        multiplicity of digital subband signals, and wherein the at        least one transmitting section combines the digital subband        signals to obtain an output signal for transmission. The        repeater system comprises    -   a power profile estimation unit which determines a power        estimate for each digital subband signal associated with the        multiplicity of subbands, and    -   a muting device which mutes a digital subband signal of a        particular subband based on the power estimate.

The advantages and advantageous embodiments described above for thedigital repeater system equally apply also to the method such that itshall be referred to the above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic overview of a digital repeater systemcomprising a host unit connected to multiple remote units;

FIG. 2 shows a schematic drawing showing functional processing blocks inthe host unit and a remote unit for digital processing of an RF signalin a downlink direction;

FIG. 3 shows a functional diagram of a digital signal processor on theside of the host unit and on the side of the remote unit;

FIG. 4 shows a schematic drawing of a frequency band including multiplesubbands; and

FIG. 5 shows a complex digital subband signal including an in-phasecomponent and a quadrature component.

DETAILED DESCRIPTION

FIG. 1 shows, in a schematic drawing, a digital repeater system 1comprising a host unit 10 and multiple remote units 11. The host unit 10may for example be located outside a building 4 and comprises an antenna100 via which the host unit 10 is for example connected to one ormultiple base stations of one or multiple communication networks usingan air interface. The remote units 11 are located on different floors 40of the building 4 and serve to provide coverage throughout the differentfloors 40 of the building 4. The remote units 11 are connected to thehost unit 10 via a transport medium 12, for example a network of opticalfibers, and hence are in communication connection with the host unit 10.Each remote unit 11 comprises an antenna 110 via which the remote unit11 for example is connected to a mobile device of a user on a floor 40of the building 4.

In a downlink direction D, an RF input signal RF_(IN) is received viathe antenna 100 at the host unit 10, is processed within the host unit10 for transporting it via the transport medium 12 to the differentremote units 11 and, after further processing, is send out via theantennas 110 as an RF output signal RF_(OUT). The RF output signalRF_(OUT) may be received by a mobile device in the building 4.

Vice versa, in an uplink direction an RF signal received at the antenna110 of a remote unit 11 is fed via the transport medium 12 to the hostunit 10 and is retransmitted via the antenna 100 towards an outsidecommunication network.

The digital repeater system 1 performs a digital processing of the RFsignal, as functionally shown in FIG. 2. FIG. 2 herein depicts suchfunctional entities of the host unit 10 and a remote unit 11 connectedvia the transport medium 12 to the host unit 10 which are used in thedownlink direction D for processing the RF input signal RF_(IN) forretransmission.

In the embodiment of FIG. 2, the host unit 10 comprises a receivingsection 20 for processing an RF input signal RF_(IN) received by theantenna 100. In particular, the receiving section 20 comprises an RFfilter 201 in the shape of a bandpass filter for filtering out afrequency band to be processed and transported for retransmission. Fromthe RF filter 201 the RF input signal RF_(IN) is fed to a low noiseamplifier 202 and to a downconverter 203 for downconverting the RFsignal into an intermediate frequency band. After that, the signal isfed to an analog-to-digital converter 204 for digitizing the RF signal,such that an (intermediate-frequency) digital signal IF_(IN) isobtained.

The digital signal IF_(IN) is fed to a digital signal processor (DSP)205 and is digitally processed in the digital signal processor 205, asshall be described in more detail below.

After digital signal processing, the processed signal is, via thetransport medium 12, transported to a digital signal processor 206 of atransmitting section 21 of a remote unit 11, in which the signal isfurther processed such that a digital signal IF_(OUT) in theintermediate frequency range is obtained. This digital signal IF_(OUT)is converted to an analog RF signal by means of a digital-to-analogconverter 207, is upconverted by an upconverter 208, is power-amplifiedby a power amplifier 209, is filtered by an RF filter 210 in the shapeof a bandpass filter and is transmitted as an RF output signal RF_(OUT)via the antenna 110 of the remote unit 11.

In the uplink direction U, essentially the same takes place, wherein inthat case an RF input signal is received and processed by a receivingsection 20 of a remote unit 11 and is further processed and transmittedvia a transmitting section 21 of the host unit 10.

FIG. 3 shows a functional block diagram of the digital signal processor205 of the receiving section 20 and the digital signal processor 206 ofthe transmitting section 21.

The digital signal processors 205, 206 serve to isolate differentsubbands contained in the overall frequency band of a received RF signalfor processing the subbands in an isolated, channelized form by digitalprocessing, wherein the subbands my overlap (for example by 50%, or maybe distinct in frequency and in this case do not overlap.

In an example shown in FIG. 4, a frequency band B, for examplecorresponding to the complete 3GPP frequency band, may comprise multipleseparate subbands S1-S6 which are separated from each other in thefrequency domain and each of which is associated with a particularcarrier defined by its carrier frequency. The subbands S1-S6 hence areseparated in the frequency spectrum of the frequency band B, such thatby separating portions of the frequency band B from each other thesubbands S1-S6 and the signals contained therein may be processed in anisolated fashion.

In this regard it is to be noted that the subbands not necessarilycorrespond to actual carrier frequencies. Generally, a broadband signalmay be split into several narrowband subband signals, the subbandsignals corresponding to frequency portions of the overall broadbandsignal.

The digital signal processor 205 of the receiving section 20 comprises amultiplicity of demodulators 301 which serve to demodulate the differentsignals associated with the subbands S1-S6 into the baseband. Thedemodulators 301 output complex digital subband signals comprising anin-phase component I and a quadrature component Q which together make upthe complex digital subband signal. After the demodulator 301 thecomponents I, Q of the digital subband signal associated with a subbandare fed to digital lowpass filters 302, 303 which have a bandwidth BW₁corresponding to the bandwidth of the associated subband. After thedigital filters 302, 303 the components I, Q are fed to downsamplers304, 305 in which the sampling rate of the components I, Q of thedigital subband signal is decimated to the minimum acceptable value suchthat after passing the downsamplers 304, 305 the components I, Q have acomparatively low sampling rate.

The components I, Q of the digital subband signal associated with asubband are then fed to a power profile estimation unit 306, which isconfigured to compute an estimate of the momentary power contained in adigital subband signal. For this, the power profile estimation unit 306,at a particular sample time, forms the sum of the squared sample valuesof the in-phase component I and the quadrature component Q of thedigital subband signal, which gives an estimate of the power containedin the digital subband signal at the sampling time.

In principle, the square root of the sum of the squares has to becalculated to calculate the actual power. But calculating the squareroot in general is costly. Therefore, for implementing a simplethreshold detector the desired threshold is squared and compared againstthe power profile.

This is illustrated in FIG. 5, showing the in-phase component I and thequadrature component Q as a function of time t. The in-phase component Iand the quadrature component Q generally have the same frequency andamplitude, but are 90° out of phase. The in-phase component I and thequadrature component Q together make up the (complex) digital subbandsignal associated with a subband, wherein the in-phase component I andthe quadrature component Q represent digital signals digitally sampledat sampling times t_(i). The (momentary) power contained in the digitalsubband signal at a sampling time t_(i) equals the sum of the square ofthe sampling values of the in-phase component I and the quadraturecomponent Q.

Based on the momentary power estimate determined in the power profileestimation unit 306 for each digital subband signal it is then decidedwhether to mute a digital subband signal or not. This decision may forexample be taken by comparing the power estimate to a pre-determinedthreshold. If the power estimate of a particular digital subband signalis smaller than the threshold, then it is assumed that the digitalsubband signal does not carry data, but only noise, and hence it isdecided to mute the digital subband signal.

However, also a more complex decision may be taken by detecting forexample interfering signals or intermodulation products and mutingsignals relating to such interfering signals or intermodulationproducts.

Based on this decision, in a muting device 307 comprising switches 308,309 the digital subband signal is muted or not. If it is decided to mutethe digital subband signal, the in-phase component I and the quadraturecomponent Q are set to zero by switching the switches 308, 309 to alower position and hence disconnecting the digital subband signal. If itis decided not to mute the digital subband signal, the switches 308, 309take the position as shown in FIG. 3.

The threshold may be expressed in terms of a minimum signal-to-noiseratio. For example, if the signal-to-noise ratio (SNR) for a digitalsubband signal is smaller than 6 dB, it may be decided to mute thedigital subband signal.

The decision whether to mute a digital subband signal or not may betaken for each sampling time for which a power estimate is available.The decision hence may change in a rather fast manner for each samplingtime.

After the muting device 307 the components I, Q of the digital subbandsignal are fed to a multistage aggregation, transport and distributionblock 310 in which the different digital subband signals are aggregatedto form a combined signal, are transported via the transport medium 12and are distributed to the different remote units 11. In the multistageaggregation, transport and distribution block 310 also a multiplexingand serializing of the digital subband signals which are not muted maybe carried out.

In this regard, only such digital subband signals are transported viathe transport medium 12 which are not muted. Hence, if a digital subbandsignal is muted, it is not transported via the transport medium 12, suchthat it does not add to the noise of the overall system and reduces thespectrum of the transported signals.

The multistage aggregation, transport and distribution block 310 isimplemented in part by means of the digital signal processor 205 on theside of the receiving section 20, in part by the transport medium 12 andin part by the digital signal processor 206 on the side of thetransmitting section 21 and indicates, in a simplified form, suchprocessing which is done on the digital subband signals for transportingit from the host unit 10 to the remote units 11 in the downlinkdirection or from the remote units 11 to the host unit 10 in the uplinkdirection.

After transportation, the digital subband signals stemming from themultistage aggregation, transport and distribution block 310 are fed toupsamplers 311, 312 in which the in-phase component I and the quadraturecomponent Q of the separate, isolated subband signals are upsampled byinterpolation or by feeding in zeroes. After digital filtering indigital filters 313, 314 for removing images arising from the upsamplingthe components I, Q of the different digital subband signals are fed tomodulators 315, which serve to modulate the (complex) digital subbandsignals to produce a real digital signal for each subband in theintermediate frequency range. Such real signals are fed to a combiner316 and are combined to a combined digital signal IF_(OUT) in theintermediate frequency range. The digital signal IF_(OUT) is then, asshown in FIG. 2, converted to an analog signal, is upconverted,amplified and transmitted as an analog RF signal.

In this regard it also is possible that from the transmitting section adigital signal is sent out via a digital link for example to a basestation hotel in the context of a distributed antenna system in theuplink direction.

Because only such digital subband signals are transported via thetransport medium 12 which contain a certain power level and which hencecan be assumed to carry useful data, the overall system performance isimproved in that noise or other unwanted signals of such digital subbandsignals are muted and not transported. Hence, for example the totalnoise of the system may be reduced.

The decision whether to mute a digital subband signal herein can betaken in a momentary fashion based on each power estimate for eachsample available. The muting decision carried out in the muting device307 hence may be varying in a fast manner. In addition, the mutingdecision may also be fed to the multistage aggregation, transport anddistribution block 310 such that the network for transporting thedigital subband signals may be reconfigured based on the mutingdecision. Herein, the reconfiguration of the network may be carried outin a slower fashion, for example only if a decision whether to mute achannel or not does not change for a pre-defined number of samples.

In the uplink direction the processing on the side of the transmissionsection 21 (FIG. 3) is performed at the host unit 10. Herein, multiplesubbands from multiple remote units 11 are processed and combined in thecombiner 316. The combining may take into account the selection ofsubband signals by the aggregation unit 310 such that only those subbandsignals of one or multiple remote units are combined for routing to oneor multiple base stations of one or multiple outside networks whichcontain useful information and which have not been muted. This mayimprove the SNR in the combined signal.

The combining herein may also take a mapping to an antenna port of abase station into account such that, for routing a signal to aparticular antenna port of a particular base station, only those subbandsignals are combined which are mapped to the particular antenna port.

In this regard the transmitting section 21 may also include multiplecombiner devices 316 to produce multiple combined signals for routing,in the uplink direction, to multiple antenna ports of one or multiplebase stations of one or multiple outside telecommunications networks.

The idea underlying the invention is not limited to the embodimentsdescribed above, but can be implemented also in an entirely differentfashion.

For example, it in principle is not necessary to process the digitalsubband signals in the baseband, but it would also be possible toprocess the signals in an intermediate frequency band.

In addition, the bandwidths of the separate subbands do not necessarilyhave to be equal, but could be different for the different subbands. Forexample, a subband corresponding to a GSM carrier may have a differentbandwidth than a carrier corresponding to a UMTS or LTE carrier, whereinsubbands may also overlap in the frequency domain.

In this regard, the frequency band processed by the repeater system ingeneral may contain different subbands associated with different radiotechnologies such that via the repeater system different kinds ofsignals can be processed and transported.

LIST OF REFERENCE NUMERALS

1 System

10 Host unit

100 Antenna

11 Remote unit

110 Antenna

12 Transport medium

20 Receiving section

21 Transmitting section

201 RF filter

202 Low-noise amplifier

203 Downconverter

204 Analog-to-digital converter

205, 206 Digital signal processor (DSP)

207 Digital-to-analog converter

208 Upconverter

209 Power amplifier

210 RF filter

301 Demodulator

302, 303 Digital filter

304, 305 Downsampler

306 Power profile estimation unit

307 Muting device

308, 309 Switch

310 Signal aggregation and transport

311, 312 Upsampler

313, 314 Digital filter

315 Modulator

316 Combiner

4 Building

40 Floor

B Frequency band

D Downlink direction

f Frequency

I In-phase component

IF_(IN), IF_(OUT) (Intermediate-frequency) digital signal

Q Quadrature component

RF_(IN) RF input signal

RF_(OUT) RF output signal

S1-S6 Subbands

t Time

t_(i) Sample time

U Uplink direction

1. A digital repeater system for transmitting signals, comprising: areceiving section for processing a digital input signal, the digitalinput signal comprising at least one frequency band including amultiplicity of subbands associated with a multiplicity of communicationchannels; at least one transmitting section for transmitting an outputsignal, wherein the receiving section is constituted to isolate, withinthe digital input signal, the multiplicity of subbands from each otherto obtain a multiplicity of digital subband signals, and wherein the atleast one transmitting section is constituted to combine the digitalsubband signals to obtain an output signal for transmission; a powerprofile estimation unit for determining a power estimate for eachdigital subband signal associated with the multiplicity of subbands; anda muting device for muting a digital subband signal of a particularsubband based on the power estimate.
 2. The digital repeater system ofclaim 1, wherein the receiving section is constituted to receive an RFinput signal and to digitize the RF input signal to obtain the digitalinput signal; and wherein the at least one transmitting section isconstituted to combine the digital subband signals to obtain a digitaloutput signal and to convert the digital output signal to an RF outputsignal for transmission.
 3. The digital repeater system of claim 1,wherein the muting device is constituted to mute a digital subbandsignal if the power estimate for the digital subband signal is smallerthan a predetermined threshold.
 4. The digital repeater system of claim3, wherein the power profile estimation unit is constituted to determinethe power estimate as a function of time, and the muting device isconstituted to mute a digital subband signal of a particular subband ifthe momentary power estimate for the particular subband is smaller thanthe predetermined threshold.
 5. The digital repeater system of claim 1,wherein the receiving section comprises a multiplicity of demodulatorsfor demodulating the digital input signal into the baseband to obtainthe digital subband signals, each digital subband signal comprising anin-phase component and a quadrature component.
 6. The digital repeatersystem of claim 5, wherein the power profile estimation unit isconstituted to determine the power estimate at a particular digitalsample time by adding squared digital sample values of the in-phasecomponent and the quadrature component.
 7. The digital repeater systemof claim 1, wherein the receiving section comprises a multiplicity ofdigital filters for digitally filtering the digital subband signals. 8.The digital repeater system of claim 1, wherein the receiving sectioncomprises a multiplicity of downsamplers for downsampling the samplerate of the digital subband signals.
 9. The digital repeater system ofclaim 1, wherein the muting device comprises at least one switch forswitching of a digital subband signal associated with a particularsubband in order to mute the digital subband signal.
 10. The digitalrepeater system of claim 1, wherein the receiving section is connectedto the transmitting section via a transport medium.
 11. The digitalrepeater system of claim 10, wherein prior to transmitting the digitalsubband signals via the transport medium, the digital subband signalsare fed to an aggregation unit for aggregating the signals to betransported via the transport medium.
 12. The digital repeater system ofclaim 11, wherein the aggregation unit takes a muting decision intoaccount in that the aggregation unit aggregates only digital subbandsignals for transport via the transport medium which are not muted. 13.The digital repeater system of claim 1, wherein the transmitting sectioncomprises a multiplicity of modulators for modulating the digitalsubband signals and a combiner to combine the modulated digital subbandsignals to obtain the digital output signal.
 14. The digital repeatersystem of claim 1, wherein the digital repeater system comprises: a hostunit; and at least one remote unit remote from the host unit andconnected to the host unit via a transport medium.
 15. A method foroperating a digital repeater system for transmitting signals,comprising: processing a digital input signal, the digital input signalcomprising at least one frequency band including a multiplicity ofsubbands associated with a multiplicity of communication channels;isolating, within the digital input signal, the multiplicity of subbandsfrom each other to obtain a multiplicity of digital subband signals;determining, by a power profile estimation unit of the system, a powerestimate for each digital subband signal associated with themultiplicity of subbands; muting, by a muting device of the system, adigital subband signal of a particular subband based on the powerestimate; combining the digital subband signals to obtain an outputsignal for transmission; and transmitting the output signal.
 16. Themethod of claim 15, wherein the digital input signal is processed by areceiving section of the system; and wherein at least one transmittingsection of the system transmits the output signal;
 17. The method ofclaim 16, wherein the receiving section isolates the multiplicity ofsubbands from each other to obtain the multiplicity of digital subbandsignals.
 18. The method of claim 16, wherein the at least onetransmitting section combines the digital subband signals to obtain theoutput signal for transmission.
 19. The method of claim 15, wherein apower profile estimation unit of the system determines the powerestimate for each digital subband signal associated with themultiplicity of subbands.
 20. The method of claim 15, wherein a mutingdevice of the system mutes the digital subband signal of the particularsubband based on the power estimate.